1. Field of the Invention
The present invention relates to a method of production of a steel soft nitrided machine part comprising hot forging a part, then cutting or otherwise machining it without thermal refining or normalization or other heat treatment, then soft nitriding it, in particular relates to a method of production of a steel soft nitrided machine part suitable for the production of a crankshaft or other machine part.
2. Description of the Related Art
In general, machine parts used in automobiles, industrial machinery, construction machinery, etc. are made by shaping S45C or other carbon steel materials for machine structures defined by JIS G4051 by hot forging, then thermal refining and normalizing or otherwise heat treating it, then cutting or otherwise machining it for finishing work. Further, among such machine parts, in particular crankshafts and other machine parts where a high fatigue strength and wear resistance are required are further soft nitrided, high frequency quenched, carburized, or otherwise heat treated for surface hardening in addition to the above treatment in the final process. Among these surface hardening treatments, soft nitridation has the advantage that the heating temperature is a low one of 600° C. or so and the heat treatment strain is small. However, soft nitridation gives a hardened layer of a shallow depth, so also has the problems of the small effect of improvement of the fatigue strength compared with the case of high frequency quenching or carburization. For this reason, a method of production of a steel soft nitrided machine part giving a machine part with a high fatigue strength is being sought.
Therefore, in the past, to cut costs and improve productivity, there has been proposed a steel material able to improve the fatigue strength and other mechanical properties and bending correctability or other workability even if omitting the thermal refining or normalizing after hot forging and even with soft nitridation (for example, see Japanese Patent Publication (A) No. 2002-226939, Japanese Patent Publication (A) No. 2001-131687, and Japanese Patent Publication (A) No. 11-62943). For example, the non-thermal refined steel for soft nitridation described in Japanese Patent Publication (A) No. 2002-226939 establishes suitable contents of C, Mn, Cr, s-Al, Ti, and O so as to improve the strength, nitridability, and fatigue strength and establishes a suitable relationship between the O content and the Ti content and the relationship between the O content and the N content so as to suppress growth of old austenite grains at the time of hot forging and improve the bending correctability.
Further, in the non-thermal refined soft nitrided steel parts of Japanese Patent Publication (A) No. 2001-131687, even if performing soft nitridation without thermal refining or normalization, a fatigue strength equal to or better than that of normalized steel is obtained by defining the contents of C, Si, Mn, P, Cr, Ti, V, N, Al, Pb, S, and Ca in the composition of the steel material before being worked and defining the ranges of the values of Fn1 to Fn3 found from the contents of C, Mn, and N. Further, Japanese Patent Publication (A) No. 11-62943 discloses a non-thermal refined crankshaft using a steel material of a composition establishing suitable contents of C, Si, Mn, Ti, Al, N, S, Ca, P, Cr, and V.